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What is the possibility of technology like Google's balloon reducing overall connectivity and internet costs?
What is the possibility of technology like Google's balloon and its Facebook equivalent replacing satelite technology for internet access in aircraft given the rise in the next tech revolution on the internet of things, aircraft will soon be among those always connected devices and possibly autonomous? Are there regulatory impediments and would one such project on a massive scale congest air traffic?

Well remember, the cost to provide it and the price you pay aren't all that connected. The airlines will come up with a pricing model that maximizes their take. Given that they have a captive bored audience, they can charge a fair amount.

cheaper internet could be positive and a technological advantage for systems like GPS and navigation

@zeta-band: It's not just the airlines.  The people owning the satellites also will charge whatever the traffic will bear.  I think the question is better suited to Economics.

I doubt it, let's take a look at Project Loon from X (previously Google X):
The balloon is to be flown at an altitude of 20km (about 65,000 feet). The balloons cover 5000 square kilometers (about 1930 sq miles). So the balloon and its service (red triangle) kind of looks like this:

The full length of (a) is 60,000 meters (60km). Assuming the ground area is circular, the area of service at the ground (b) is about 94.9km in diameter. So you need to figure out what the service area is at 40,000 feet (about 12.2km). I won't go over the math here (that is for a different section of SE), but you get:
Beam Angle at the balloon: 134.2°
Length of (d): 37km
Service area at 12.2km: 1075 sq km

So the problem here is that the balloons would have to be spaced much closer since the (d) of the balloons would have to overlap to give continuous service. At the speed of an airliner (550mph, or about 885kmh) the airliner would speed through the 37km wide circle (assuming it takes a straight line through the center) in about 2.5 minutes. This is either hindered or helped by the balloon speeding through the stratosphere (Project Loon's record is currently 162kmh). So you would pass through anywhere from 3.1 minutes (with the balloon) to 2.1 minutes (against the balloon) worst case.
So looking at it from purely an "is it possible to provide internet service by balloon to airliners" perspective, I would say no, not economically anyway. You would need roughly 3 times the number of balloons to cover the same area at altitude as you would be on the ground. The balloons are designed for ground service, so if airplanes did use them, they would have to switch back and forth from satellites and balloons every 2-3 minutes.


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Post MH370, are there developments to improve the tracking of airliners?
Post MH370, are there developments to improve the tracking of airliners? Is there any technology being developed that would utilize the smartphones of the passengers?  MY question include smartphones and inflight wifi being standard or at least looking to be standard in the near future

The issue with MH370 wasn't tracking the aircraft's position, it was in tracking it after all the systems that enable such tracking had been shut down by pulling their circuit breakers inside the aircraft. Dilemma: provide a CB, and a rogue crew can turn the system off. Don't have a CB, and a bad box that starts smoking (electrical fire) can't be isolated. Systems that can transmit up to satellites draw lots of power, so you need the ability to isolate them. It's a dilemma, and cell phones won't solve it -- if it can reach a satellite, you do NOT want it in your hand!

@RalphJ there are a lot of handheld satellite communication devices, like sat-phones and PLB's. They are relatively low power devices.

No, no technology has been developed that utilize the smartphone of passengers because the airplane is at a location where it is outside the connectivity of any smartphone during the entire flight, except for maybe the first and last few minutes.
A smartphone needs a cell tower to establish network connectivity, which typically has a range of a few hundred meters or at most a few kilometers. Once a plane takes off and climbs to around 5000 feet (which takes only a minute or so), it is out of range of all cell towers. There are departure and approach paths which flies over populated areas, but most flies out to rural areas for noise reasons. And if a plane is over a populated city, you don't need smartphones on board to track it anyway.
The only hope of communication in the middle of an ocean is a direct connection to a satellite. Smartphones are not equipped with the necessary hardware to do that, and I doubt they will in the near future. To connect to a satellite, you will need to know where it is in the sky, which in turn means you need to know your location to a reasonable accuracy and an antenna to point the signal at the right direction.

Your idea about inflight Wi-Fi is an interesting one. Theoretically, if someone's cellphone is configured to report its location once in a while (to help in a situation where the phone is stolen or lost), and that cellphone is connected to internet, it can be used to track a plane's location. However, bandwidth onboard is limited to the Mbps to Kbps range, since it is a satellite connection. In fact it is so limited that you have to paid (quite an expensive price) for it.
Moreover, this solution does not solve the connectivity problem, because the phone needs to rely on the plane's satellite connection to establish connectivity. But there needs not be a phone - the mere act of connecting to a satellite is already enough to track a plane's location.

However, there are changes in the aircraft systems that will allow airliners to be tracked more accurately. For example, ICAO recommends that by November 2018, all aircraft over open ocean should report their position every 15 minutes.

Are you able to elaborate on the ICAO recommendation for reporting every 15 minutes? Are pilots radioing in their position (seems tedious), or is it automated via some mechanism?

@SnakeDoc the ICAO recommendation does not specify how to implement it, to allow freedom to fulfill the requirement using any suitable technology. In practice it is an automated process using satellite connectivity. Most airliners already have satellite communication capability in the cockpit (which allows the pilots to communicate with headquarters during the flight), so the hardware is already there.

source: Stack Exchange

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sort: votes


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From which jet engine is this blade?
Some years ago I bought a compressor blade on ebay but I don't know which jet engine it's from.

It's quite light. I tried to determine the density by the help of my kitchen equipment and it's most likely made of aluminum. Density 2.something g/cm3. Not even close to titanium 4.5 g/cm3 - even considering my home made volume measurement;)

From one of it's markings (JR24660A)
I found out the engine is from Rolls Royce. Other markings are:
5013 and 30815 (hand graved)
But I can't figure out the exact type. I read at wiki that old engines had indeed aluminum blades e.g. RB108. Pinhole in the foot might point to Conway or Spey? Unfortunately, good pics at Google are very limited for easy web research.
I hope someone can help me.
Thanx in advance ;)

Without all markings on the blade -- ideally good photos of them -- there's little chance of even narrowing down the engine family.

Should we ask why you bought a single compressor blade on ebay :D

Just as I wrote. There is a marking JR24660A, which is a part number. With this number I already found several companies that would purchase me a spare blade. I'll make some additional photos of the other markings.

@Jamiec the size of my desk is not sufficient for a collection of complete jet engines :)

My guess it is a titanium alloy, not aluminium (little higher density than aluminium). It is probably an old engine, the past 20 years I've only seen dove tail joints.

@Jamiec why not?  I didnt get it from Ebay, but I value my Concorde engine fan blade quite highly amongst my collection.

It's a fan blade from a Rolls Royce Spey. The part number is also confusingly mentioned as a blade for an Allison TF-41 used on LTV A-7 Corsair II's but those blades appear to be different, and they have what looks like a little airfoil about halfway up the blade.

is the airfoil presence stage-dependent, or all stages would have it?

@Federico That's  a good question. I could only find general pictures of the fans on the A-7 version and the Spey, not the other stages. I added an image link so you can see the difference.

@Juan Jimenez Thanx alot for this information. The fact that the part no is is designated for Rolls Royce Spey and Allison TF41 makes totally sence. Both companies made the TF41 as co-desing based on the Spey engine.

@Frederico The 'airfoil' on the blade is a snubber or clapper or part span shroud. This feature has several functions: it changes the natural frequencies of the blade to higher values, additionally it prevents the de-twisting of the blade during operation. It's normally used for 'long' blades with small chord length typically the fan / front stages compressor blades.
Due to the aerodynamic losses recent engines use no snubbers but hollow blades with bigger chord.

@Rednose If I were you I'd give it a mirror polish, sharpen it to a razor edge and use it as a prosciutto slicer. :)

The "little airfoil" on the TF41 Fan Blade is the blade clapper. The TF41 Fan Blade part numbers were: 6869628, 6878181, 6897200, 6892422.

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Can you get records of the actual sale prices of aircraft?
I'm doing a little market research on aircraft I'm interested in purchasing, and I'd like to know what aircraft of similar type are selling for. I pulled FAA registration records on a handful, but almost always sale price is either left blank (which I believe is actually not formally allowed) or contains "$1 + OVC" as in the photo below.

Are there other practical ways to find out what aircraft are actually selling for?
I know there are many companies offering bluebook values, but if the raw data on sale prices aren't out there, I don't know that I trust their valuations either.

See the question Why do people put $1 & OVC on aircraft bills of sale? for more information on this convention.

Have you looked at the prices on "aircraft for sale" websites?

www.controller.com

@DJClayworth, yes I have, but that is just the asking price. Presumably the aircraft could have sold for a different (probably lower) value.

@Jpe61, is there a way to view past (sold) listings on Controller? Trade-a-plane seems to give sellers the option to mark their listing as sold rather than remove it, which can be helpful (though, again, it will only reveal the asking price). I haven't found a way to view past listings on Barnstormers or Controller.

I haven't actully used that service for years now, it simply was the first one that popped into my mind, so can't really advice with the use much...

Welcome to aviation.SE! I suspect that the data is most likely to come from state records. Most states charge some form of tax on aircraft purchases, although it can depend on the value, whether it's a private or business purchase etc. In other words, it's complicated and the best way to get reliable data is probably from a specialized data provider. That might not be helpful if you're looking for free data on light aircraft sales for your own personal use, but I don't think that the data exists in any central, public location.

Good to know, @Pondlife, and thanks for the welcome! I was looking for the free option :) And also just wasn't sure I could trust a specialized data provider if the data if there is no reliable way to get the data.

Only auction listings will show price realized after the sale. Private sales are not required to put anything into the FAA bill of sale because the FAA has no taxation or other authority related to the transaction, other than to record that the aircraft changed hands.

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Prediction of flight delay. What kind of information is available to a company at the moment of scheduled departure of a flight?
I am working on a data science task of trying to predict the departure delay of flights, but I don't have much knowledge of aviation itself. I have a dataset that contains some information of the flights as origin and destination airports, airline, scheduled time of departure, etc. But unfortunatelly they seem to have very few predictive power on delays.
I have also a set of five variables that do predict somewhat the delays. They are 'Carrier Delay', 'Weather Delay', 'NAS Delay', 'Security Delay', 'Late Aircraft Delay'. But I don't have additional information on what they represent. I found this link  https://aspm.faa.gov/aspmhelp/index/Types_of_Delay.html that explains somewhat. However It is not clear for me which one, if any, of these variables I can use, since I dont' know if I would have any of these informations at the moment of scheduled departure. Therefore, using them would be cheating somehow. What do you think?
Sorry If this question doesnt belong here. I just thought that maybe someone here would know something about this kind of information.
EDIT: In this case, 'I' would be the airline itself. The exercise is to make a model that would use all the data that a company has available till the moment of scheduled departure to predict the delay.
On my dataset I have different information, as the actual arrival delay, which naturally is very correlated with the departure delay. But I cannot use it naturally, as I don't know the arrival delay at the moment of scheduled departure. Doing so would be cheating. My question is whether using these 5 variables above would be also cheating.

Are “you” the general flying public? The pilot/company? Some other entity?  Obviously, “insiders” would have different access to different variables compared to the general public

"I" would be an insider, specifically the airline.

@ymb1 I added some information

@ymb1 not exactly. I want to predict departure delay of future flights. In my dataset I have information of departure and arrival delays of past flights, among other info. What I say that I assume I cannot use information of arrival delay to model departure delay since it happens later.

@ymb1, yes it is indicative. But if I want to use my model in real life, I can not predict the departure delay by using information that I ll only have in the future, which is the case of arrival delay of the same flight. I need to use only information that I have prior the prediction moment

You’ll have EDCT and GDP info at departure time, plus your own delays due to the inbound flight arriving late, missing/timed-out crews or maintenance. You’ll also know the ETE for the route you’ve chosen (usually to avoid weather), though ATC may change it.

You may want to clarify how far ahead you are attempting to predict. “At the moment of scheduled departure” is too late; if you are NOT leaving at that very moment you will know exactly why because all the information will be available to you right then, and no amount of prediction will prevent it. Also, different factors are known at different times. For example, if the airplane you are using hasn’t departed on the prior leg you will know hours prior. But an unruly passenger at door closing, last second security delay...

Are you trying to predict based on known current conditions: weather, airport conditions etc  or are you trying to use statistical analysis of past flight data to make the prediction? Or a combination?

Airlines track delay rates (leaving the gate more than 15 minutes after the departure time) as "chargeable" and "non-chargeable" (chargeable, as in "blame-able" you might say, to the OEM), to separate causes that are the airplane's fault and causes that are the airline's fault or due to other external factors.
Chargeable delays are due to mechanical issues that a related to the airplanes own reliability.  These delays are considered the responsibility of the Manufacturer to address with design changes or maintenance guidance.  The industry standard for Chargeable delays (because something on the airplane broke) is not more than 1% of departures, generally expressed as Dispatch Reliability of 99% or better.  A lot of airplanes are less than that, but under about 97 or 98 % is starting to look pretty bad and a mature design that is well executed should have no problem maintaining 99% or very close to it.
Whatever the number is, at a mature airline flying a mature design, the number will be fairly consistent over time and you can make assumptions about future delay rates based on current delay rates or their trends.
Non-chargeable delays are those that are the airline's fault (airline-induced maintenance, delays leaving overnight maintenance, weather delays, ATC-related delays that show up in the overall dispatch reliability or schedule completion data but are not the fault of the airplane itself).  These delays are also carefully tracked and at a mature operator the numbers should be fairly consistent, notwithstanding crises that arise out of nowhere and skew the numbers from time to time.
So all those delay categories mentioned in your post would be tracked by the airline and maintained in an internal database and used to forecast various things, like the need to keep spare aircraft that sit on standby to fill in for cancellations, that sort of thing.
So to answer the specific question, the airline will maintain a detailed database of departure delays for all causes, in order to be aware of and act on, to the extent it can, the source of the delays, to keep its fleet wide departure delay and cancellation rate, and schedule completion rate, within a specification.
Schedule completion rate is the key number at the end of it all, a measure of the success in getting passenger X from point A to point B, eventually.  Spare aircraft kept on hand to fill in when flights are cancelled won't help with technical delay rates, but will greatly improve the Schedule Completion Rate.
There will be an internal reliability engineering organization at the airline responsible for tracking and acting on the information, and making forecasts for future requirements to head off problems before they come up.


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Do most aircraft have a ditch button in the case of emergency?
In the ditching of US Airways Flight 1549 in 2009, the pilots did not have time to press the "ditch" button present on the Airbus aircraft they were flying. In other circumstances where time permits, the "ditch" button may help slow the rate of flooding if the aircraft remained intact after impact. The "ditch" button seals off all valves, inlets, and opening underneath the aircraft, to prevent water from entering through these places. My question is, do all large, commercial aircraft have this button? For example, do Boeing 737, 747, 767, 777, and 787 have this switch? Thanks for any help!
image of the ditch button on the Airbus A330 from Wikipedia

Format your text!

@Cloud Can you please tell me how to format my text? Which part do you think should be re-formatted? My content isn't too long, so do you think I still need to separate them into two paragraphs? Thanks for suggestions!

Can downvoter please tell me how to improve? I'll appreciate any suggestions. Thanks in advance!  :)

I've only ever seen this button on Airbus aircraft.
It's hard to prove a negative, but as an example here is the ditching checklist for a Boeing 777, where the outflow valves are closed manually and no ditching button is involved:

Ditching
Condition: Airplane ditching and evacuation are needed.

Plan to jettison fuel as needed to reduce VREF speeds.
Checklist Complete Except Deferred Items

Deferred Items
Below 5,000 feet
GND PROX GEAR OVRD switch ... OVRD
GND PROX TERR OVRD switch ... OVRD
PACK switches (both) ... Off
OUTFLOW VALVE switches (both) ... MAN
OUTFLOW VALVE MANUAL switches (both) ... Hold in CLOSE until the outflow valve indications show fully closed [...]
On final approach (omit Landing checklist)
Landing gear lever ... UP
Flaps ... 30

(Boeing 777 QRH - 0.1 Ditching)
For reference, on the Airbus A320 the ditching button closes more than just the outflow valve:

The operating system sends a "close" signal to the outflow valve, emergency ram air inlet, avionics ventilation inlet and extract valves, and pack flow control valves.

(Airbus A320 FCOM - Air Conditioning and Pressurization - Pressurization - DITCHING guarded pushbutton)

Thanks for your answer! So basically only Airbus integrated all these procedures into one simple button. I think other aircraft should have this too because time is extremely crucial in emergency situations and needing to spend precious seconds in manually closing valves and stuff defers the start of the actual ditching as well as distracting the pilots. Apparently, Airbus did a great job on this.

@AircraftEnthusiast007, as with many safety things having this as a single control also means a single fault, software error, mis press or other issue can trigger a whole bunch of events that turn an otherwise safe flight into a complicated emergency, so is not an automatic win. See for example helicopter ejector seats which require having a method to deliberately remove the rotor system.

@AircraftEnthusiast007, ditchings of jet airliners are extremely rare -- about one a decade.  Ditchings on short notice are even less common: US Airways 1549 is the only one I've been able to find.  Optimizing for extremely rare events is generally not a good idea.

@Mark And ditchings of large aircraft generally go very badly anyway.  1549 was special in that they could ditch in a river rather than a large body of water and thus had a much lower threat from waves.

The comments miss a point: the Airbus ditch switch isn't just for ditching. Its most common used purpose is to seal the aircraft when deicing to prevent fluid ingestion, as well as some maintenance tests. It's marked for ditching because that's the most safety and time-critical use. Optimizing for the most critical use of a control is a important human factors concept.

@GremlinWranger I agree with you that in many cases integrating too much into a single button/switch may not be the best choice since it can be a mistake. But in this case I don't see how closing valves and openings can cause too much trouble even if it is not intended. Even if it does cause trouble, then it would've been relatively easy to solve. In comparison, if an aircraft is really going for an emergency  ditching, having such a button would save the pilots' precious seconds before impact. Thus, I think having this functionality would be better than not having it.


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How can I obtain forgotten A&P certificate numbers?
I lost my A&P years ago after leaving the airlines and do not remember the certificate numbers.
I got my licenses around 1999. Does anyone know the process for obtaining the certs?  (Do not remember if the FAA used SSNs as cert #'s back then).

According to this the FAA stopped using SSNs for certificate numbers in 2002.  Although, my recollection is that there was a transition period beforehand (maybe it was optional to not use it for a few years?).  So, there's a good chance that it is your SSN.
In any case, you can get in touch with a local FSDO and they should be able to issue you a new ATP card, as well as provide other information, once you identify yourself (SSN, prior address etc)

Thank you for the advice. I was trying to process it online since I'm currently residing overseas, but will make it a point to visit a FSDO next time Im in the US.


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Examples of unmanned airports?
I just found this sentence on Wikipedia:

In aviation, some possibly unmanned airports have equipment that let pilots key their VHF radio a number of times in order to request an Automatic Terminal Information Service broadcast, or turn on runway lights.

What kind of airports are unmanned and what other important features do they have, from the perspective of a pilot?

Welcome to Aviation.SE! This is a very broad topic. We already have a bunch of questions on non-towered airports. Is there something specific you want to know about them?

@Bianfable quite frankly, I left it intentionally broad because I can't really visualise the concept. What I'm looking for is basically the breadth of the conditions in which people choose to create an unmanned airport as opposed to a manned one.

Note that Wikipedia's terminology is incorrect; airports without an operational control tower will not have an ATIS, though they might have an AWOS or ASOS broadcast. See my answer here explaining the difference.

Welcome to aviation.SE! For context, the US has over 19,000 airports and only 520 have control towers. So non-towered airports are a lot more common than you might think, although obviously if you look at passenger volumes etc. the towered airports are much more visible and important.

Any decently-sized empty field/paddock, beach, or empty stretch of road could qualify as an unmanned airport, in an emergency ?

As someone who came from HNQ: perhaps the question title could be clarified that this is more about operating airplanes? At first, I thought it's also about passenger/commercial airports that are fully unmanned.

Reasons to leave the airfield unmanned
The common reason is the cost of providing services. For a small airfield without commercial activity, it makes sense to have no control tower and no controller. This is also true for non commercial helipads. Such airfields are known as non-towered or non-controlled airfields, they are not the same, but for the purpose of answering your question, we can consider them as without personnel. Many airfields are only part-time controlled, during day and uncontrolled at night.

Uncontrolled airfield, source
From Wikipedia:

When the traffic volume at an airport gets too high for safe and efficient operations, or when the mix of aircraft types and speeds becomes too large, an airport may be considered for a tower.

However, it is also necessary to find the money to construct a building and pay the controllers' salaries; in some cases aviation regulations or local opposition may prevent establishment of the unit.

Pilots will coordinate their activities in the airfield perimeter by announcing what they do, e.g. where they are, or their intent to land, on a common frequency, the common traffic advisory frequency/CTAF in North America, or any other frequency assigned for this purpose.
Non-towered airfields are very common. From AOPA:

Nontowered airports—those not served by an operating air traffic control (ATC) tower—are much more common than towered fields. In fact, nearly 20,000 airports in the United States are nontowered, compared to approximately 500 that have towers.

Remote towers
Some airfields may be remote-controlled, controllers are assigned but are not at the airfield tower. This can be done using cameras and virtual towers. This is the case of London City Airport (EGLC). This is similar to how container cranes in ports can be remotely controlled from low cost centers.

Remote controller for EGLC, source: NATS
From Wikipedia about a remote virtual tower:

The main benefit of RVT is expected to be cost efficiency. The cost savings originate from the following factors:

No need to build and maintain control tower buildings and facilities at the local airports.
The building and operational costs of a remote tower and facilities are much lower compared to a traditional tower.
More efficient use of human resources (ATCOs and AFISOs), especially by serving multiple airports with medium to low traffic levels from a centralised location.
Reduced need to establish and maintain ATM systems locally at the airports. By using data communication networks from the local airport to the remote tower centre, several technical systems can be centralised, hence costs savings are possible. Over 75% of regional airports with fewer than 1 million passengers a year are making a loss.11 These costs could be shared, saving 1.3 million euro per year (in case of Shannon and Cork airports being controlled from Dublin).

Regulation
In the US, FAA regulations (e.g. AC 90-66) apply to these airfields.
See also: Uncontrolled Aerodromes - Procedures at SKYbrary
Pilot activation
On airfields with no personnel (including no remote personnel), some devices may be operated using predetermined schedules or by remote-control by pilots. This includes runway approach and runway lighting, ATIS transmission (including weather information) and navaids.
Remote orders are decoded from the VHF frequency (CTAF) carrier transmission and interruption sequence. Such systems are known as pilot-controlled lighting (PCL), pilot-activated lighting (PAL) or aircraft radio control of aerodrome lighting (ARCAL). From ACAMS, a provider for such remote activation systems:

If "everybody" needs to coordinate use of untowered/uncontrolled aerodromes - and there's no dedicated channel for just the specific aerodrome - doesn't that mean a lot of radio chatter and over-talk, at least potentially?

@einpoklum: You're right, it can happen. However there are possibilities to change the frequency. Still such CTAF is used for short blind messages, not for conversations.

@einpoklum isn't that why high-traffic ones aren't uncontrolled? also you may be thinking radio goes an unlimited distance - it does not

Small private airstrips are typically “unmanned”. Many small municipally owned airports are “unmanned”.
These small airports often have no lighting. Many do have lighting and it makes sense to have an automated system for lights or ATIS.
It is mainly an economic decision to have an “unmanned” airport. An airport with little traffic can still operate safely without a control tower and full time staff.

The more correct term here is an 'untowered' airport. As others have mentioned, untowered airports are by far the majority of all airports, with tens of thousands of them in the U.S. alone. Untowered airports don't have a tower generally because they just don't need one.
There are specific procedures that pilots follow when flying in or out of an untowered airport, including a frequency that pilots use to talk to other pilots flying in and out of that airport, such as for announcing our position and intentions. There is also a specific traffic pattern that aircraft will fly to approach the airport so that other traffic knows where to expect traffic to be. For the lower levels of traffic that are typically present at these airports, these procedures work fine without the need for an air traffic controller with a radar.
I say that the more correct term is 'untowered,' because untowered fields are often not actually unmanned, especially public use ones during the daytime. All of the fields I flew out of during flight training were untowered airports, but all of them had staff on hand during the day, for example, in a Fixed-Base Operator (FBO) office. These are the people who rent the hangars and tie-down parking, sell fuel, and possibly also provide other services such as aircraft rental, flight instruction, aircraft maintenance, and small supply shops for pilots. Many also provide facilities like bathrooms, kitchens, seating areas, etc. for use by pilots. In many (probably most) cases, the FBO is truly unmanned at night, but usually still has some degree of accessibility to pilots flying after hours who need to fuel their aircraft or use other facilities provided by the FBO.
Untowered airports also often provide automated weather reporting services via something like AWOS or and ASOS. Runway lighting is typically controlled by clicking the mic a certain number of times on the radio frequency used by pilots at that airport to talk to each other. The runway lighting system also monitors this frequency and turns on/adjusts the lights based on pilots transmitting those 'codes.' The lights generally turn off again after a certain amount of time passes in order to save energy and lengthen the lifespan of the lights when there's no one needing them.
As far as what types of fields are untowered, these can range from literally just a strip of grass in some pilot's field being used as a private airport to helipads to sea plane bases to public-use airports with runways long enough for medium-sized jets. Pretty much the only types of airports that are usually not untowered are airports with scheduled commercial passenger service, military bases, and unusually busy general aviation airports that have too much traffic to operate safely without air traffic control, such as the GA airports in major metropolitan areas.

There are many untowered airports with scheduled passenger airline service. Not on the level of ORD or LHR, of course, but they exist.

@randomhead True, though they're much more the exception than the rule. Technically, even Coldfoot, Alaska (population something like 12) could be said to have scheduled service (though it's part 135 instead of 121,) but it's a Piper Navajo with flights cancelled when not needed (or when weather not VFR because there's no radar out there. - haha) Definitely no tower there.


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What is the minimum set of registers to fill for ADS-B?
I am researching about Automatic Dependent Surveillance-Broadcast mode of Mode S currently. On a third party document, I encountered a list of transponder registers to be filled; 05, 06, 07, 08, 09, 0A, 61, 62, 63, 64, 65 as the list of registers.The source is Grappel 2007- ATC 334.
05(Airborne position), 06(Surface Position), 08(Aircraft Id), 09(Airborne velocity) registers seem to be carrying most important data for ADS-B.
Registers from 61 to 65  are called event driven registers, carrying slowly changing data. SHALL these registers have to be filled with correct values to serve ADS-B IN capable systems? What happens if I don' t service these event-driven registers?
Is there a definitive list of registers to fill current settled ADS-B standards?
Note: I have access to many documents, so feel free to refer some well-known ADS-B documents.

ADS-B IN systems do require information from the 0x6X registers. These registers hold information like ADS-B protocol version, system design assurance level and position source quality indicators.
Without this information the ADS-B IN system can establish a track, but the quality will be flagged as too low to display.
For reference, get a copy of RTCA DO-260B or EUROCAE ED-102A, whichever is cheaper. They are the Minimum  Operational  Performance Standards for 1090 MHz Extended Squitter Automatic Dependent Surveillance – Broadcast  (ADS-B).

The MOPS describes several classes of ADS-B equipment, not all of which are applicable.  You may get some help in picking the right class by reading TSO-C166b as it includes specific FAA guidance regarding classes and functionality.

Gerry, what do you mean with "not applicable"? Perhaps you mean not applicable in the US. However, the TSO you refer to is not applicable outside the US. For all ADS-B specifications the TSO refers to DO-260B. The standard functioning of ADS-B IN, and the necessary registers are described in the MOPS, not in the TSO.

DO-260B describes a number of classes of ADS-B equipment.  The minimum data set required is dependent on the class the manufacturer desires to certificate the equipment to.  Note that Class A equipment must support both ADS-B IN and OUT.  Class B is ADS-B OUT only.  There are other classes for ground vehicles.  While the TSO only covers FAA approvals, there is an equivalent ETSO for EASA.  DO-260B and ED-102A are harmonized documents so the rules are the same. ADS-B OUT data standards depend on the chosen Class.  ADS-B IN data support can be 'universal' or limited to defined applications.

Looking back, it's not clear whether the Mehmet is discussing the design of the ADS-B link or it's installation.  For installation, information is available in AC 20-165B for ADS-B OUT and AC 20-172B for ADS-B OUT.

I was asking about ADS-B OUT data coverage to be exact. Could we "get away" with not updating certain ADS-B allocated registers. Would ignoring some registers anger ATC?

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Do you need to put your wings at the center of gravity when building an airplane?
I have been wondering about this for quite a while and can't find an answer.

For supersonic speed, putting the wings near the center of gravity would be a good start. Even better, place the wing's center slightly aft of the center of gravity in a conventional configuration.

Lockheed F-104 in flight (picture source)
If you use rotating wings, placing their roots right above the center of gravity has worked well, too, especially if there is only one attachment point for them.

AS 355 Ecureuil helicopter (picture source)
If there is more than one attachment point, however, the rotating wings should be at opposite ends of the fuselage, quite a bit away from the center of gravity.

Boeing-Vertol CH-46 helicopter (picture source)
At subsonic speed it is advantageous to stretch the wings out sideways, so at least their tips are again quite a bit away from the center of gravity.

Nimeta single-seat glider (picture source)
And then there are some odd designs which try to separate the wings from the center of gravity as much as possible, and they still fly.

Rutan Proteus tandem wing (picture source)
So there is no general rule. Maybe these answers help to answer your questions about wing placement relative to the center of gravity.

Absolutely not, CG is a variable.
The wing creates a center of lift (can be refered to depending on what you're trying to achieve as center of pressure or aerodynamic center).
CG works in a partnership with the center of pressure and the horizontal stabilizer to give the aircraft it's stability characteristics.

Would be good to mention the typical relation between Cg and the wings / weight and balance to give a "full" answer.

@Notts90 agreed, feel free to add to it as I have already passed my ATPL mass and balance exam 3 months ago!


text:
What is the most commonly occurring fault with this type of electrically driven gyro?
I have an examination tomorrow about advanced avionics systems (EASA module 13) and we are required to troubleshoot a fault with the compass system. What is the most commonly occurring fault with a gyro compass system and how is it detected or identified?

It would help if you stated the fault. In my military a/c maintenance gyros failed for mechanical (things like gyro bearing failures) and electrical reasons (chafed wire, etc.).

Some idiot using magnetized a read by flux valve.
Gears  slipping. Sync knob sticking in.
Leaving it in DG mode.so it drifts
Not correcting for Magnetic Variation changes unless you fly along an isogonal


text:
How were the graveyard spiral and other sensory illusions discovered?
The graveyard spiral has an infamous reputation for killing even experienced pilots which only have visual flight experience. What the wikipedia article does not mention is how and when the spiral and other dangerous sensory illusions were discovered.
Essentially it means that pilots in the beginning years would be found inexplicably killed by seemingly harmless weather with visual impairment (fog, night). Someone must have survived such an experience and lived to tell the tale. Is there any information about the hunt for the cause of early crashes and when they identified the culprit?

I'd guess that fairly early in the history of aviation, pilots got 'the leans' (mentioned in the Wikipedia article) in IMC, emerged into VMC and realised what had happened?

Yes, but then it should occur sooner or later in letters, aviation/medical prints (what existed at that time) and further discussions. In comparison: We very well know the point of realization about the dangers of metal fatigue: The de Havilland Comet incidents. We also know that humans can survive incredible amounts of g for very short timeframes because Dr. Stapp investigated it und undertook himself dangerous experiments. So why is so little known about the discovery of sensory illusions?

This paper from the website of the Department of Behavioral Sciences and Leadership at the US Air Force Academy reviewed cases dating back to 1913 (note though it doesn't assert that the reports in 1913 suspected Spatial Disorientation (SD)). In the linked paper the author references a previous paper which I can't seem to find: Gibb RW . Historical assessment of visual spatial disorientation
[abstract] . Aviat Space Environ Med 2010 ; 81 : 318 .

In 2010, the lead author presented an assessment of visual spatial
  disorientation at the annual Aerospace Medical Association conference
  and cited 25 studies dating from 1947 to the present declaring SD’s
  role in mishaps as well as surveys of pilots anonymously sharing their
  SD experiences

The earliest research they reference is a Navy survey from 1947. Quoted from the linked paper

1947: U.S. Naval aviators, 67 total, reported on their illusionary
  experiences and categorized them into visual, non-visual, conflicting
  sensory cues, dissociative, and emotional ( 48 ). Of note within
  visual illusions categories were confusion with lights, depth
  perception, “ black night, ” and judging height above the
  ground/water. Experienced pilots were still prone to illusions
  regardless of total flight time.
( 48 )Vinacke WE . Illusions experienced by aircraft pilots while
  flying .J Aviat Med 1947 ; 18 : 308 – 25 .

The linked article suggests that the aviation community has known about these issues since the early days of flight but the issue was (and they assert still is) an underrated and underreported flight risk which may skew reporting which causes underfunding. Thus the published research may lag the time in which the community started noticing SD.


text:
Is a license required to transmit in the designated aviation band?
Is an FCC license required for people to operate a radio in the aviation band?

just need to be clear about the FCC rules

Welcome to aviation.SE! Are you asking about radios in aircraft, on the ground, or both?

I assume you want to know for people that have a legitimate need to transmit, as indeed if you not involved in a flight activity, you have no right to use the aviation reserved band for transmission.

When I first learned to fly, I was told that R in the ARROW  mnemonic (https://aviation.stackexchange.com/questions/37657/which-regulations-require-the-arrow-documents/38149) had been a requirement for an FCC radio license in the US until around the early 1980s.  But I don't have a proper cite to confirm this.

This is a bit of a tangent, but I once tried to find the regulations prohibiting an airline passenger from transmitting on airband frequencies, using a call sign such as "Boeing 228UA". I didn't find any such regulations, so I'm not 100% sure that that's actually illegal! (Of course, common sense says it's a bad idea...)

The FCC does not presently require a license for Aviation Radio installations in aircraft, where those are VHF and UHF (as I recall).  These are covered in Part 87 of the FCC regulations.
Travel outside the US requires a station license, and I believe HF transmissions require a license.
Similarly the operator (pilot) needs a permit when traveling outside the US, but does not need the permit within the US, to answer your question about the operator.

Within the Continental US, no, VHF air band comms do not require a license.  You are required to possess a restricted radiotelephone operator’s permit ($80 and some online paperwork with the FCC) when using command over international waters and while flying in foreign nations.

When you are the PIC (Pilot in Command) you of course are granted permission to operate those systems as needed as it’s required to properly and in most airspace classes legally fly the craft.
If you own/volunteer/operate at an FBO with it’s own FCC assigned frequency and/or your airport has no control tower and is assigned a ‘Unicom’ frequency (e.g. 122.80 MHz) then you can transmit as needed to dispatch WX info, conditions, etc.
If you have nothing to do with this field or are an avid flight simmer heh..(yes I am too despite flying in real life), then it is illegal to TRANSMIT IN ANYWAY on aviation frequencies (VHF or HF). A license to purchase one is NOT required however.
NOTE:  Some sellers will not sell you an aviation radio (transceiver) without you showing them your license or other authorized certification). That is up to the dealer and is perfectly okay.
The FAA & FCC DOES require you to be Certified (authorized anotherwords) to modify, install, remove, calibrate and ‘sign off’on any piece of avionics equipment involving communications and among other equipment, navigation equipment.
Obtaining certification(s) to install, repair, fix & calibrate 2-way comms (handheld, FBO or in the plane) for aviation can be obtained by attending a certified school or company that offers training & certification.
There are several different types of certifications offered depending on the scope of what you want to do. The basic ‘technician’ can  remove/replace in dash comm systems, however, CANNOT by law troubleshoot it beyond that of the most obvious of adjustments on the front panel switches/options, etc.
Another type of certification is required to actually open it up and bench test it, replace components and calibrate it. It then requires someone to ‘sign off’ on the repairs made and certify it to be ‘air worthy’ so to speak so it can be re-installed. It yet again needs to be signed off on post installation.
Tim Dickerson, CEM, FCC/FAA ACC.
ARS N9NU
Woodlake Landing (Sandwich, IL)
Unicom: 122.80. ICAO:  IS65
Commercial License -135 Endorsed


text:
Why do pilots sample the fuel before a flight?
I was reading this checkup writeup and it says the pilot took fuel samples before the flight. So I Googled and found out, there are several fuel samplers/testers available.
Now I am wondering: Why is this necessary? Is it a mandatory procedure or do you just do this if you don't trust the supplier?
I can also imagine, that you check for water or contamination. However, we never do that with our cars.
Or am I completely off track?

As a partial answer, on many cars, especially diesel cars, you have a fuel filter with a bowl or a water separator that helps prevent water from entering the engine.  Cars also have pretty good designs to prevent water from entering the fuel tank and fuel cap.

With cars you don't do a forced landing if the engine dies due to water ingestion.

@GdD You are right. However, a stall on the Autobahn due to engine failure isn't not dangerous. It has happened to me on the very left lane (of three) into road works without a shoulder to stop. I had to keep rolling without servo steering until there was one. But I get the point. It is not comparable with a car. ;)

@CodyP If that was the case, then why would airplanes not have the same good design. It is way more crucial than in a car I guess.

On the first flight of the day, after every refueling, and if you have flown through precipitation or the plane is left in the rain, you check the fuel for water and to make sure you have the right kind of fuel. Since water is heavier than fuel, if there is just a little bit of water in the fuel it could cause the engine to stop. You can’t pull over to the side of the road like you can in a car, so you need to immediately deal with the problem. The most likely place for this to occur is on takeoff. If you think fast and switch tanks, you might get lucky and only have water in one tank so you can continue your takeoff. If not, you may need to find a place to land.
In 17 years of flying, I have only found water in the fuel once, and it wasn’t even my airplane. But since it was on Catalina Island, a stopped engine would not have been pretty.
The other reason to check the fuel is that airplanes designed for 100LL do not burn Jet Fuel. Jet fuel is denser than 100LL so it will sink to the bottom of the tank. It is straw colored and somewhat greasy, so you can tell if your plane has accidentally been filled with it, but you really have to be paying attention. It would be easy to miss if you just topped off. There have been incidents when twin-engine planes have mistakenly been filled with Jet A and have either crashed or required expensive fuel system cleaning. Mike Busch's article describes his experiences. After the misfueling of a Cessna 421s, someone did a test to see how easy it is to identify a combination of Jet A and Avgas. It’s fairly hard using standard pre-flight practices. That’s why many people recommend watching the fuel truck if you have a twin-engine plane or one that prominently says “Turbo” on the side.
Many aircraft have rubber bladders to hold the fuel. Most older Cessnas  have them. Water can get trapped in the folds of the bladder and get into the engine later when the fuel is sloshed around.
Even if the fuel source is free of water, a tank that is not full can have water condense in it.
Most autogas has some ethanol in it. Water will dissolve in a gas-ethanol mixture and so a slug of water won’t get into the engine like it will in an aircraft engine.
Contamination is less of an issue and is one that is really hard for a GA pilot to determine. You really can’t tell if the fuel is contaminated with other petroleum products just by looking at it.

Let's say that my aircraft is re-filled with jet fuel instead of 100LL without any special equipment how can I realise it before flight? and there is any special equipment to buy (not expensive please) to bring with me during my flights? thank's

@Ghilardi, fuels are dyed by the manufacturers exactly for this purpose. 100LL is blue. So if the sample has wrong color, or the color is faded, something else was mixed in and you should drain it and refuel fresh.

@Ghilardi As mentioned in the linked Mike Busch article it is difficult to see a color difference if a small percentage of the fuel is jet fuel. One sure way to determine whether jet fuel is in your sample is to put some on a paper towel. 100LL will evaporate quickly and leave no residue. Jet A leaves an oily residue.

Jet fuel also has a distinctly different smell.  It would be a good idea to learn to distinguish the two.  I bet the local fuel supplier would be happy to give you a sample sniff if you asked.

Here is an example of a plane being misfueled and crashing:  http://www.kathrynsreport.com/2014/08/inmates-in-bright-orange-jumpsuits-load.html

*Even if the fuel source is free of water, a tank that is not full can have water condense in it.
Most autogas has some ethanol in it. Water will dissolve in a gas-ethanol mixture and so a slug of water won’t get into the engine like it will in an aircraft engine.* What is the solution to this? As we know, our car sometimes, or even most time, the tank is not full. Maybe just half. So, how to solve the water inside the tank?

An anecdote, if I may.
Once when I was a student pilot, I was preflighting a Cessna 152 for a solo flight. I always sampled the fuel before a flight, as I had been taught. On this day, I noticed a tiny red speck in the sample from the right tank. Tiny, like maybe the size of a grain of sand, but red. It bothered me. I tried another sample and got a red flake, still small. The plane was white with red trim. A paint flake, perhaps? I got up on the wing, but there was no red trim near the fuel filler, nor was there any near the drain. I pulled the cap, thinking it might be from a deteriorating seal. The seal was black and in good condition. I tried the left tank and it looked fine. Tried more samples from the right tank and got a red flexible blob. At that point, I took the sample inside and asked the chief mechanic. He immediately grounded the plane.
As I recall, the determination was that someone had transferred fuel to and/or from the plane using a hose that wasn't made for gasoline. The lining of the hose had come out and into the tanks. Both tanks had gobs of the stuff, though I had found it in samples from only one. The plane had shop time logged just before my preflight, so it had probably happened then.
I was grateful I'd done a proper preflight inspection and paid attention to that little red speck. Had I not, the engine might have run long enough for me to get airborne before that stuff plugged up the fuel screens. I could just imagine being a few hundred feet up and faced with the "impossible turn" as a student pilot. Not a pleasant thought.

Sampling fuel is a mandatory procedure. It's purpose is to check if water has gotten into the fuel tanks.
There are many ways which water can enter a fuel tank: Heavy rain , an unsecured fuel cap, fuel contamination are some of them.
Cars are not sampled because an engine failure on a car is a minor event. On a plane it is a major emergency, and if you are in a single-engine aircraft you will become a glider.
A story from my instructor: In Mexico there was a situation where someone wanted a person dead. Remember these were real, professional assassins. The fuel tank was filled with water during a turn-around on the ground, and the pilot did not check fuel samples. The plane crashed minutes after it took off.

Mandatory by what authority? It's a good idea but is there a legal requirement to do so?

The Front Cover of my Pilot Operating Handbook is marked "THIS HANDBOOK INCLUDES THE MATERIAL REQUIRED TO BE FURNISHED TO THE PILOT
BY FAR PART 23". Later on: "The limitations included in this section have been
approved by the Federal Aviation Administration."  In the Checklists "Before first flight and after each refueling, use sampler cup and drain small quantity of fuel from fuel tank sump quick-drain valve to check for water, sediment."   So reads like it's sorta mandatory by the FAA. If you filled up with water and the NTSB determines you didn't check and thereupon crashed, it's on you.

I was flying yesterday, couldn't get a sample from my right tank. Had to wait for temps to climb to melt what I guess was water frozen around the drain valve. When it finally flowed, it took several samples before I got one to come out water-free.  Not sure where it came from; wash water after a recent repainting? Water in the fuel at the painting airfield? (Had some water there before leaving as well). Water in the fuel at my home field? Water from rain over the weekend? Plane is going in for an oil change, having the gas cap seals changed to new at the same time, just in case.


text:
What is this plane, based on its inside shape?
I'm not sure my question is valid here but would be very helpful to me.
Here is the picture of the inside of a plane (don't mind the ostrich). Does anyone know which plane it is? The flight number was 4357 going from Marseille to Frankfurt in 2006. I think the plane was from Lufthansa but not 100% sure. Can't find any public database for consulting historic flights.

The 2+2 seat configuration would encompasse Embraer and Bombardier.

On this website => http://uk.flightaware.com/live/flight/DLH4357, it says it was a 737-500 in 2010, is it possible it was a plane like that back then ?

B737-500 is wider 3+3. Could also be an ATR-72 (just trying to match the 2+2 seat configuration)

This 2006 Lufthansa timetable lists '143' as scheduled on the Lufthansa 4357 flight from Marseille to Frankfurt. But 143 would be Avro RJ85 which has a 3+3 configuration. If this is indeed a Lufthansa (Cityline) plane in 2006, it would be a Bombardier CRJ-100, -200 or -700 (or maybe the -900, they got delivered from June 2006). The Embraers were not in service yet with Lufthansa in 2006.

As addition to that, the JetPhotos database has a photo of a CRJ-100 at Marseille taken on 15 July 2006, so it's likely this plane type visited multiple times

You guys can provide some precious infos ! Thanks :-)

A 737 is 2+2 in first class or business class.

@mins The cabin looks too wide for a 2+2 to me (for a size reference, look at the heads of the people seated in the frontmost rows and compare them to the width of the cabin), and the upper-right corner of the photo shows a three-seat passenger service unit (although the low image quality of the photo makes this less immediately-obvious than would otherwise have been the case).

@mins: I'd actually say it's probably a 3+3, since it looks to me like the overhead bins on both sides are the same width and since the PSU on the right is definitely a three-seat unit.

Thanks for your investigation. From @Jordy comment and your observation, what are your conclusions ?

This is a 3+3: In addition to the details mentioned in the comments, there are three lights in the overhead.
And, in particular, it is an Avro RJ85. This matches the @Jordy research on the Lufthansa timetable and the details:

(right pic source)

Nice sleuthing!


text:
What PPL routes are available with gliding experience?
I'm an early solo glider pilot, with the long term goal of also obtaining a PPL (not a LAPL(A)).  I am planning to do this after I have obtained at least my bronze badge, which I aspire to within the next twelve months.
Originally I thought that the only route open to me was to do the full PPL.  My understanding was that I can get a credit of 10% of P1 hours on a glider deducted from the requirements (max 10 hrs), however the CAA website refers to holders of "a pilot’s licence for another type of aircraft, except balloons".  I'm not sure if that applies to a bronze holder or if I would need to obtain a LAPL(S) or SPL for them to count (for which I would presumably need a bronze XC endorsement to make the required XC flight).
If I obtain a LAPL(S), then according to the CAA I can add a TMG rating (6+ hrs) and then, after hours building (24hrs) do a conversion course (15+ hrs) to the full PPL(A).  It's not clear if this route is available if I obtained an SPL, although for a fee I believe you can swap licences.
Can P1 hours as a glider pilot count towards a PPL(A) without obtaining a LAPL(S)/SPL?
Is the LAPL(S) -> TMG -> PPL route viable? Would I need to go to an "ATO" for TMG training or can my local gliding club do this?
Aside from just doing a PPL from scratch, are there any other viable routes that I've missed (e.g. via a LAPL(A) on a TMG somehow)?

if you can't get an answer here, you might try rec.aviation.soaring, where lots of UK glider pilots hang out

I have a partial answer after more research and some help on pprune.
A Bronze + XC can be used to get a gliding licence, which under the transitional arrangements can be used to obtain a LAPL(S).
In theory the route from there to a LAPL(S)+TMG  and on to a LAPL(A) TMG is feasible, however the BGA is not an ATO yet and there are very few instructors or examiners around.  This route may be problematic at the moment, but should become easier as time goes on.  There's no reason not to claim the LAPL(A) on the way to a PPL(A) if taking this route.
An alternative route is via an NPPL under the transitional arrangements.  After gaining the gliding certificate, a minimum of 10 hours under instruction is needed until reaching the standard for an NPPL SMLG.  After short conversion training, this can then be converted to an NPPL SSEA.  After 6 hours PIC this lot in turn can be traded in for a LAPL(A) SEP+TMG.  Complicated, but overall less hours and my much can be done at a BGA club with a motorglider and instructor.
Regarding the straight PPL(A) route with credit, given the statement

If you hold a pilot’s licence for another type of aircraft, except
  balloons, you will be credited with 10% of your total flight time as
  PIC up to a maximum of 10 hours

it would seem clear that at that some form of licence is needed, and given the gliding licence qualifies for a LAPL(S) anyway, which one is moot.


text:
Is a Mode S address changed when an aircraft is sold to another country?
The 24 bit Mode S address is issued by the national CAA where the aircraft is registered.
If it is sold to another country, with another CAA, is the Mode S address maintained or is a new one assigned?

A new Mode S address will have to be assigned by the new authority:

Aircraft addresses will be assigned in keeping with the following principles:

[...] when an aircraft changes its State of Registry, the previously assigned
address shall be relinquished and the new registering authority shall assign it
a new address; [...]

(ICAO - Guidelines and considerations for standaradised registration of aircraft with Mode S transponders)
Until the new authority has assigned a new address, ICAO may assign a temporary address to the aircraft:

Temporary addresses will be assigned to aircraft in special circumstances when operators
have been unable to obtain an address from their States of Registry or private common
mark registering authorities on a timely basis. ICAO will assign temporary addresses
from the ICAO block shown in the appendix to this paper [...]

(ICAO - Guidelines and considerations for standaradised registration of aircraft with Mode S transponders)


text:
Can pitot tube measure boundary layer? Why?
Can pitot tube measure boundary layer? Why? Why not?

Welcome to Aviation.SE! I don't quite understand what you are asking. A pitot tube is measuring pressure. What do you mean by "measure boundary layer"? Do you mean measure inside the boundary layer?

To measure the boundary layer (BL) mean velocity profile the most used techniques are two:

Use of hot wires: these techniques are subject to the error that can be caused by the body thermal conductivity or the changes in the air conditions and should be calibrated before the experiment (usually with a Pitot tube in clean air);

use of Pitot tubes : these techniques requires corrections for near wall and shear effects, and in the most of the case also because of the structure  elastic effects.

For what it concerns the local thickness of the BL, you can use one of the definitions by analysing the boundary layer mean velocity profile.
You can find a nice article about these techniques here or refer to the "bible" of experimental fluid mechanics here.


text:
How do gas turbine engines and turbofan engines keep constant compressor side rpm?
How do the RPMs stay constant in a gas turbine engine? For example in car with piston engine, when you keep pressing the accelerator the rpms keep picking up, to keep the rpms constant you must let off the throttle, if you keep the throttle floored you will reach peak powers rpm and even go higher rpm if engines max rpm isnt limited at peak power. My question is that how do gas turbine engines can stay at peak power rpm while still throttle is fully applied? Shouldnt it keep increasing in rpms like car engine?

"in car with piston engine, when you keep pressing the accelerator the rpms keep picking up" -> when I do that in my car in highest gear on the Autobahn, the RPMs will eventually stop increasing before reaching peak power and I've reached my top speed. I think your premise is somewhat flawed, probably by always driving your car well below top speed.

@Bianfable In autobahn its because majority of cars top gear are overdrive. you will reach your top speed way before you reach peak power in top gear because rpms are so low. If you are on 1st and 2nd gear etc you will easily reach peak power rpm and keep going up from there still until you hit the rev limiter. My cars engine makes peak power at 6000 rpm but if i keep pressing accelerator it easily goes to 6500 rpm which is the limit. I added the constant speed prop tag because i thought maybe the answer would be because they are similar but i really dont know

Modern engines use https://en.wikipedia.org/wiki/FADEC. Here's a video explaining the implemtation that's quite good.  https://nescacademy.nasa.gov/video/135553bc3b7b4171b7c54ee0578489211d

ALL these engines will runaway, if load is abruptly removed
What keeps RPM at sensible numbers is that usable power (load) is kept balanced with the fuel supplied.
There are a number of X-factors depending on the application.
A turbofan is going through thick air, which places considerable load on it.  The fan is directly coupled to the N1 shaft.  It would be extremely unlikely for the fan to unexpectedly start ingesting the vacuum of space, and that's what would be required for the load to suddenly be removed.
Now the N1 shaft could break and you bet that would cause an overspeed event.  Similarly a helicopter or turboprop transmission could snap, or the prop could simply be feathered the wrong direction.   In that case you have some sort of rev limiter, or a proper governor that changes  fuel flow to try to reach and stay at the target RPM.
Most diesels and small engines have these governors built right in.  You don't actually work the throttle on a bulldozer or lawnmower engine, you are telling the governor to seek a different target RPM. The fuel rack sticking wide-open is death to a diesel.
With gas cars, someone could jab the clutch or shift the automatic into neutral. In automobiles you have a simple rev limiter that knocks out the spark, though these days they like to knock out fuel also. You would never have that on an airplane, but absent a feathering mistake a runaway is unlikely due to that thick fluid the prop is always pushing. Same for a marine waterscrew.
With stationary turbine generators like the LM2500, the reactivity of the AC power grid is going to force it to the target RPM no matter what it does. The only option the turbine has is to push, or be dragged lol.  However if it suffered a generator trip where the grid connection was abruptly removed, it will immediately overspeed unless controls cut off the fuel.
In fact, this was seen in The China Syndrome - it started with a generator trip due to a grid issue. To prevent the steam turbine from overspeeding, the turbine tripped (cut off steam), so suddenly there was nowhere for 2.5 gigawatts of thermal energy to go, so the reactor SCRAMmed to stop that at the source. Jack felt a shudder and the story begins.

The reason that car piston engines and turbine engines seem to behave differently is that they're used in different ways.
If ran at full throttle with no load, both types of engine would accelerate to their maximum RPM (limited either by internal friction or by a fuel control unit). However, if you place either type of engine under load, they will both accelerate more slowly, and it will take much longer to reach that point.
Airplane engines and car engines are used differently. During takeoff, the engine is usually accelerated to full throttle as soon as the takeoff roll starts, and is left there until the airplane has sufficient altitude. This is plenty of time for it to reach its maximum RPM, and stay there. On the other hand, cars have a much more restrictive speed limit than planes (relative to what their engines can do), so the driver usually takes their foot off the accelerator long before the engine has enough time to reach its RPM limit.
This is easily seen by looking at the behavior of piston airplane engines. When I take off in a rented Cessna 172 with a piston engine, I advance the throttle to max, and the engine rapidly climbs to its maximum RPM and stays there. It doesn't continue to accelerate, even though the "accelerator" is floored. (And it doesn't have a fuel control unit, or indeed any kind of computerization at all.)

Yeah but why does it stay there? For example in car if you floor it in 1st gear you will soon reach the max rpm and hit the rev limiter. if rev limiter didnt exist it would keep revving even more and damage the engine. The ecu just cuts off fuel to prevent overrevving but this also means its also no longer making as much power because fuel delivery is now decreased. It will just bounce off the limiter. Do plane engines also limit fuel delivery to keep it at steady rpm? If so, isnt that basically decreasing their power output because now there is less fuel? Sorry my question is a bit confusing

@TimothyScherer It stays at that RPM because the load on the engine precisely balances the power output. As RPM increases, the wind resistance of the propeller increases. Eventually, you reach a point where the engine can't produce enough power to overcome the resistance, and the RPM stabilizes at that point. A car in first gear presents a very low load to the engine, which would be roughly equivalent to a piston plane engine with no propeller attached. In those situations, the plane engine would behave very much like the car-in-first-gear engine: if you have the throttle up,

it would over-rev until it either hits the rev limiter (if it has one) or until it suffers damage (if it doesn't have one). On the other hand, if it does have the propeller attached, it behaves analogous to a car in high gear, where the engine will eventually reach a balance point between the load and power before it hits the limiter.

Also, yes, the fact that there's less fuel at max RPM limits the power output of the engine, whether it's a turbine, aviation piston, or car piston. This is, in fact, why it limits the amount of fuel, because having less power is preferable to blowing up the engine, and thus having no power.

In a car, you virtually-always operate with the engine far below its power-limited RPM; this is usually not the case in a jet-engined airplane.
In both a piston-engined car and a jet-engined airplane, if you apply a certain amount of power (by pressing the gas pedal a certain distance in a car, and by moving the throttle lever through a certain angle in a jet), the engine will accelerate until it reaches an RPM where the power available at that throttle position is just sufficient to keep the engine spinning at that RPM (i.e., the amount of power needed to counter the forces trying to slow the engine down is equal to the amount of power the engine is currently generating).  Once it reaches this power-limited RPM, the engine will then operate at a constant RPM until either the throttle setting is changed (altering the amount of power available from the engine) or the forces resisting the engine's rotation change (altering the amount of power needed to counter these forces).
You can see this on a car, too, not just on an airplane.  If you sit in a car with its engine idling, and then lightly press the gas pedal, deflecting it by a small amount, and hold the pedal at this deflection, the engine (and car) will accelerate up to a certain RPM and then level off at that RPM, just like how moving a jet's throttle lever to a certain position and leaving it there will cause the plane's jet engine(s) to accelerate to a certain RPM and stay there.
The difference between cars and airplanes in this regard is that most automobile drivers virtually never use maximum throttle for more than a second or two, if that.  This is because, if you floor the gas pedal on a car and then hold it floored, the engine, and the car, will accelerate until the car is far above the speed limits on most roads before the engine stabilizes at its power-limited RPM.  The steady-state throttle setting that corresponds to the car moving at a speed that doesn't earn its driver a speeding ticket is a fairly-small fraction of the maximum throttle setting, and it's fairly-hard to hold a specific fairly-small gas-pedal deflection constant with one's foot (especially since car gas pedals, unlike airplane throttle levers, automatically return to the idle position when released, requiring one to apply a constant fairly-small amount of pressure to the pedal in order to hold a constant fairly-small pedal deflection); it's much easier to allow the engine's power (and its RPM, and the car's speed) to decay somewhat, and then, once the power/RPM/speed's fallen past a certain level, push the gas pedal down to deliver a brief pulse of higher power to reaccelerate the engine (and the car) - a pulse that is brief enough that the engine never reaches its power-limited RPM before you let off the gas pedal again.
In contrast, maximum throttle is frequently used for up to several minutes at a time when flying jets (as it provides the best margins of safety for takeoffs and go-arounds), leaving plenty of time for the engine to stabilize at its power-limited RPM.  Additionally, with jets, even when using less-than-full power, there are two major factors that make picking a certain throttle setting and allowing the engine to stabilize at its power-limited RPM for that throttle setting a much-more-practical course of action in most flight regimes than providing periodic throttle pulses (as in a car):

If you move an airplane's throttle lever to a certain position and then release it, it stays at that position (instead of trying to return to idle, as with a car's gas pedal), making it much easier to keep the throttle in the same position for long enough for the engine's RPM to stabilize.
Gas-turbine engines have much slower throttle response than the piston engines found in most cars; due to the massive rotational inertia of a jet engine's large, heavy compressor and turbine rotors, it takes much longer to speed up or slow down when the throttle setting is changed.  This slow throttle response makes supplying periodic brief pulses of power much less practical than with a piston-engined car.

What all this means:

In a piston-engined car, when you apply high throttle, you generally release the gas pedal long before the engine stabilizes at its power-limited RPM.
In contrast, in a jet-engined airplane, when you apply high throttle, you generally keep the throttle lever at this throttle setting for way more than enough time for the engine to stabilize at its power-limited RPM.

Pretty early on in the development of jet engines they realized that it was necessary to limit fuel to prevent runaway RPM.  For this reason RPM is governed by a Fuel Control Unit that meters fuel according to certain parameters such as RPM, throttle position, temperature, pressure, etc.  Even at full throttle the FCU is set up to limit top RPM.

Thank you, this is what i was looking for. Other answer doesnt make any sense. One thing i am also wondering though, since you cut off fuel supply to limit rpm to not let it overspeed, isnt that also decreasing the maximum power/thrust? For example AGT-1500 gas turbine engine in m1 abrams tank makes 1500hp peak power but if the governor  cuts the fuel to prevent it from overspeeding , would it not make less than 1500hp? Or is 1500hp the advertised hp after rpm settles with the governor cutting the fuel delivery?

This answer isn't correct. The OP was asking about the difference between turbine engines and piston engines. Piston engines are also susceptible to runaway RPM if they have no load, and thus also have some kind of Fuel Control Unit that limits top RPM.

@HiddenWindshield, the OP apparently disagrees with you.

@HiddenWindshield Yeah the ecu just cuts off fuel when you hit the rev limiter. My question was basically how did gas turbines and jet engines etc also coped with this situation of overspeeding and Michael Halls answer provided the answer.

@TimothyScherer I see. I may have misunderstood the question, then. Though, in my defense, if the question really was just "Do jet engines have FCUs?", then I would argue that it was poorly worded.

ah yes i think  i have worded it a bit poorly my bad

@TimothyScherer: can you please update your question to be more precise then? This question/answers exchange will be accessible to many people for many years to come and they also deserve to understand it

With car engines, the connection between car speed and engine speed is stiff, and only changed by changing gears or operating the clutch. So a car engine need to change rpm often to adapt to car speed and load (idling, accelerating, going up- or downhill). If kept on full throttle, rpm rise to a point that depends on the gear, mechanical losses, air resistance and road steepness.
On contrast, a jet engine rpm is connected to the speed of the airflow going through the engine. Even if the plane is standing on the ground, full throttle will result in a high airflow, giving a far less stiff connection between plane speed and engine rpm. The rpm on full throttle will finally depend mostly on air density and intake pressure, which changes with height and airplane speed. But engine exhaust will always be faster than airspeed.
So if full power is needed, full throttle can be applied and the resistance to the engine will not change that much while rolling, takeoff and initial climb.
In contrast, a car engine would go through it's gears, adapting its speed over and over again to match the cars road speed. Otherwise the tires would slip. This adaption is somehow made by soft air for the airplane case.
Today all those engines are computer controlled of course to keep them in safe operating conditions and give better economy.


text:
Is foreign based flight-time accepted in Canada?
I'm currently doing PPL course in Canada. Actually, I have flight times which was logged in my country. Can this flight time used when meeting the requirement for the PPL flight time?

Discussion of the question can be found in the Reddit thread

HT @tedder42 for the link to this redit thread which shows that ...
It may be accepted in credit towards the Canadian experience requirements.
From Canadian Flight Regulations -

Division VI - Private Pilot Licence
421.26 Aeroplanes - Requirements
(9) Credits for Foreign Applicants
(c) An applicant who does not hold a Private or higher pilot licence -
Aeroplane issued by a contracting state may be credited foreign dual
and solo aeroplane flight training time and aeroplane ground school
time towards the knowledge and experience requirements for the issue
of a Private Pilot Licence - Aeroplane, if the applicant provides
certification from the holder of a Flight Instructor Rating -
Aeroplane that all ground and flight training exercises have been
satisfactorily reviewed.


text:
Why are transponders to be turned off when in formation?
Reading through FAR AIM. I came across the ADSB section which stated that transponders should be turned off except for the lead plane. Why is this?

Because otherwise, being so close together, they would all have their transponders trigger at the same moment in the sweep, and all reply at the same moment, and all of the replies would arrive at the receiver at the same time, with the same strength, leading to a "garble" in which none of them could be reliably decoded and plotted.

Doesn't ATC also have automated systems for detecting dangerous situations? Those would lead to constant alarms for loss of separation. That might be another reason.

@JörgWMittag There is no situation where aircraft would deliberately fly that close together outside of an airshow or military use. ATC would have plenty of time to avoid the situation entirely, making the need to detect this situation pretty much zero.

This isn't really a thing.

This is indeed the reason that this rule exists, but it stems from the time of Mode A/C radars. When the radar is a Mode-S radar and the aircraft are equipped with a Mode-S transponder it wouldn't matter. The radar would Selectively interrogate the aircraft one-by-one (roll call interrogations) and there would not be any garbling of the replies. The replies to All Call interrogation may be garbled if they are not suppressed by the Lock-out protocol.

I don't doubt this answer, and I am ignorant of aviation.  But I can't help but wonder if it's only part of the story.  This answer explains why the transponders won't work well in formation, but what harm would it do?  Since the cited regulations specifically say to turn them off, I can't help but wonder if leaving them on would be bad, not just useless.

Has anyone seen a formation flight breakup? Say you're working ACME21, a formation flight of 4 F15's that want to split into individual flights. You tell ACME21, "have ACME22 squawk 5252 and come up on my frequency." The radar has NO PROBLEM tracking ACME22, even though he's right next to ACME21 (and CA goes off immediately). @hobbs I don't know if you're getting this from a book or something, but in the real world, closely spaced transponders are not a problem. Have you ever seen a stack of aircraft holding for a major airport with overlapping targets? Same thing, no problem.

@SyntaxJunkie ATC can see the lead plane's transponder because it is still on. If they all interfered ("if", because of the other comments) ATC would not see any transponder.

I got the info from technical materials (and yes, of course I'm talking about mode A/C). I suspect that the receivers have gotten better in the past several decades, which makes it less of a problem. But I still think that this is a very significant factor in why the rule was written in the first place.

@hobbs Can you please cite your information source? I'm not trying to be a jerk, but this reasoning was not the way things actually worked going back into the 1980's.

@hobbs, this rule is very new. The FAA published it quite recently in 2019, https://www.federalregister.gov/documents/2019/07/18/2019-15248/revision-to-automatic-dependent-surveillance-broadcast-ads-b-out-equipment-and-use-requirements

This answer seems wrong. Not tooting my own horn, but https://aviation.stackexchange.com/a/97329/20394 includes the relevant excerpts from the FAA's reasoning, which doesn't give any hint that it's based on garbled xponder transmissions.

In the US enroute environment, a formation flight is controlled through the flight leader, which is assigned a discrete transponder code. Controllers separate a standard formation from other aircraft by 6 miles (instead of the normal 5) to account for the additional lateral dimensions of the formation.
The enroute computer system (ERAM) will detect conflicts between aircraft, including aircraft on a nondiscrete beacon code, and display an alert on the controller's radar display. The conflict alert (CA) is in the form of flashing full data block (on the tracked aircraft) and flashing limited data block (on the untracked aircraft). Since the formation is separating themselves from one another, having CA activated is unnecessary.
If a flight is a non-standard formation, it is customary to have the trailing aircraft squawk the nondiscrete subset of the flight leader (flight leader is assigned 5251, trailing aircraft will be told to squawk 5200). This enables the controller to add the appropriate lateral separation depending on the formation size.
Anytime a trailing aircraft has their transponder on and the CA activates, the controller can suppress CA to stop the flashing, but that only suppresses CA on their radar display. Other controllers can still see the alert if they have a full data block of the formation lead (which happens every time the flight transitions from sector to sector. Another option is the suppress group (SG) function. SG will inhibit CA in the entire facility (on aircraft added to the SG list) but will not suppress CA across facility boundaries. SG would be used for non-standard formation flights when the trailing aircraft is squawking a nondiscrete code.

EDIT: Added the phrase that SG only applies to the aircraft in the SG list.

To reduce the number of false air collision warnings
Straight from the FAA justification of the new rule, https://www.federalregister.gov/documents/2019/07/18/2019-15248/revision-to-automatic-dependent-surveillance-broadcast-ads-b-out-equipment-and-use-requirements:

During formation flight, the close proximity of aircraft and their data/identification tags displayed on the radar display can, at a minimum, clutter the ATC display making it hard for ATC to determine the exact location of the aircraft to provide appropriate separation from other aircraft. Additionally, an air traffic controller will receive repeated audio and visual alerts (flashing data tag) that aircraft are within close proximity to each other. These alerts can distract controllers and redirect their attention to aircraft with approved separation and away from other instances where the controller may need to provide control instruction to maintain necessary separation. In these cases, once aircraft are “joined up” as a flight, it is in the best interest of flight safety to direct subsequent “wingmen” in the flight to squawk stand-by or stop squawk since control instructions are provided to only the lead and there are established Start Printed Page 34285separation minima from formation flights. In the instance of non-standard formation, it is general practice to have the lead aircraft squawk, along with the trail/last aircraft, a subset beacon code with altitude. In order to minimize these conflicting or overlapping data reports, this rule allows ATC to direct only the lead aircraft flying in formation to transmit ADS-B or operate his or her transponder.

(Emphasis added to highlight relevant sections.)

The whole flight is controlled as a single entity by ATC - hence all communications including transponders are only done to the lead plane.

do you have any source for the "the whole flight is seen as a single entity by ATC"?

Controlled as, yes. Seen as, not necessarily.

I can't speak for terminal radar systems (perhaps @757toga can) but in a standard formation, with the flight leader squawking a discrete code and the rest of the formation squawking standby, you will not see a primary target on the non-leader aircraft on an enroute display. As the primary radar feeds data into ERAM, the tracking software compares the primary radar tracking with the secondary radar tracking (or ADS-B data) and suppresses primary target display on aircraft that have a roughly matching secondary or ADS-B target.


text:
How do aircraft inverters work and why are they necessary?
How do inverters in aircraft work? And why are they necessary?

Aircraft may have a variety of inverters for different purposes or none at all, can you provide some more detail on what kind of inverter you are asking about? or the specific system in the aircraft where you have seen it referenced. Aircrafts can also invert themselves with sufficient control force applied...

Inverters in general. No specific aircraft.

Since you're not asking specifically about aircraft, you might search electronics.stackexchange.com. I'm sure this question has been asked there and will have been covered more thoroughly.

This is also going to help.

Inverter is a switching-circuit that takes DC as input and converts it into AC of the desired frequency and waveform.
(Note that an inverter will not change the voltage. This can be remedied by fitting a transformer downstream of the inverter.)
An inverter can be supplied with direct current from:

The battery

The TRU

TRU stands for Transformer Rectifier Unit. It takes AC input and converts it into DC of the desired voltage. It can be supplied by AC of any waveform/frequency. The supply first passes through the transformer which changes its voltage, then through the rectifier which converts it into DC output.
On most aircraft, the TRU usually provides a 28VDC output; this is because the 24 volt batteries on board require this as their charging voltage. Also, this allows the TRU output to act as battery output during normal operations.
However, modern aircraft AC systems operate at 115VAC @400Hz. This means that a transformer downstream of the inverter will be necessary to boost the voltage. The inverter converts 28VDC into 28VAC @400Hz, and the transformer then converts that into 115VAC @400Hz, which can then be supplied to the AC bus. The transformer is usually an integral part of the inverter unit.
A combination of TRU and the inverter unit can therefore convert any random AC source into AC of the desired voltage, frequency and waveform. When the engines are unavailable, the TRU is replaced by the batteries.
Some applications of the inverter
In those aircraft whose generators are not equipped with a Constant Speed Drive Unit, generator output (wild-frequency AC) is not usable as such. It must first be converted into DC supply by the TRU, then finally into a usable AC supply by the inverter.
In those aircraft whose generators are equipped with CSDU, the generator output is 115VAC @400Hz as it is, and can be directly supplied to the AC bus. However, there may be applications where a different frequency/voltage is desirable - for example, the standard 115/230 VAC @50/60Hz outlets. Here, a combination of TRU and inverter can provide this supply.
Inverter is also an essential part of circuitry required to drive synchronous motors, since they require a constantly varying frequency input during acceleration/deceleration. Here, a device called ESC is used, which combines a variable-inverter and a variable-transformer. The combination is capable of providing alternating current varying in frequency and voltage. Synchronous motors are often used as engine starters.

On a relatively simple turboprop like a Twin Otter, the output from the generators is 28V DC and this powers most of the electrical equipment on board eg fuel pumps and the hydraulic pump.
Some flight instruments require 115V 400 Hz AC power and this is supplied from inverters. After reading the above good answer, I understand that a transformer must also a part of this process to increase the voltage.
On larger aircraft the generator power is AC and most electrical equipment is powered by this. Equipment that needs DC power are supplied from Transformer-Rectifier Units which on the B767 eg are supplied by the Main AC Busses. The B767 also has a Standby AC Bus for supply to some essential items. In some situations, the ship’s batteries (DC) can power this bus through the Standby Inverter.

The B767 Standby AC Bus has other power sources apart from the ships battery(s). One of these is Right Main AC powering Right DC via TRU and the Right DC powers the Standby Bus through the Standby Inverter. AC > DC > AC.

If you want to add something after posting the answer, try using the edit feature instead of comments :-)


text:
Why is it that some planes require the AC to be turned off during engine start and take off?
I've noticed that some pilots turn off the climate control for start up and takeoff, like the Sr22. Why is this? And has this been phased out with new technology?

In the Sr22 there are no limitations to A/C operation with the factory installed automotive style vapour cycle system, except for a minor takeoff performance penalty. The compressor uses 6 HP and there is a 100 increase in ground roll with it on.

The pilots just like to mess with the passengers. You notice how they turn the lights on and off all the time, too? They think that is funny. After spending 10,000 hours in a cockpit, the threshhold for amusement is pretty low.

More or less addressed in my answer to this similar question but aircraft POH data is typically derived from full power numbers which means the engine is driving power only to the propeller and the alternator for required electrical systems for the given flight regime. Since engine power directly impacts aircraft takeoff performance you would not want something arbitrarily reducing that power at a critical time or causing another potential failure point.
Also addressed in this question and this one (not a dupe as those seem to be about jet aircraft)
Its worth noting that this is not a technical limitation its a "safety first" way of operating aircraft, certifying them, and generally thinking about flying. You can technically run climate control during takeoff nothing is physically preventing it other than your takeoff performance numbers.

Assuming there's a heat pump system involved (as with home and auto air conditioning), it draws considerable power from the engine, and is also subject to unpredicted pump seizure which can result in sudden power reduction (when it's needed most), start a fire under the cowl, or even completely stall the engine (I've had a seized pump in a car cause the engine to stall, requiring me to cut a drive belt to continue driving -- not something you can do at 13,000 AGL or fifteen seconds after rotation during a takeoff).
Turning off the climate control compressor during takeoff seems a minor precaution to prevent this possibility, and it wouldn't surprise me at all to learn that aircraft that depend on a heat pump for cabin climate control would recommend disabling that item during critical periods.
Some newer automobiles have electric motors driving accessories that used to run off the engine via drive belts (my current car has electric power steering, for instance, and electric cars, of course, have electrically powered air conditioning).  Using electric drive for a heat pump might still require shutting that heavy power draw off during startup (the battery can only supply so many amps), but would eliminate the need to keep it off during takeoff and climb out.

I can't say for sure with airplanes. But with a typical internal combustion engine car, when you start it up the air conditioning (and some other accessories, depending on the car) turn off when you move the ignition to start, and after the car starts and you move back to the run position, the air conditioning starts running.
Why? Air conditioning is a huge power drain. In fact, a car air conditioner can produce as much cold air - and use as much power - as an air conditioner for a small house (2 to 3 tons). I suspect airplanes are not much different.
In addition, in a car the concern is simply "power to start the engine". In an airplane it is "power to start the engine and to get safely off the ground". Takeoff requires more power than cruising and, as I understand it, takeoff and landing are the most dangerous parts of a typical flight, so making sure the engines have all the power they need for takeoff is critical.

Engines on some larger aircraft are started using bleed air drawn from the auxiliary power unit. This same bleed air is used for the air conditioning while on the ground without any engines running. Therefore in order to ensure a successful engine start, the air conditioning is switched off so that all the bleed air from the APU is available.